1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, wireless (contactless) power transmission has been receiving attention as a power supply technique for electronic devices such as cellular phone terminals, laptop computers, etc., or for electric vehicles. Wireless power transmission can be classified into three principal methods using an electromagnetic induction, an electromagnetic wave reception, and an electric field/magnetic field resonance.
The electromagnetic induction method is employed to supply electric power at a short range (several cm or less), which enables electric power of several hundred watts to be transmitted in a band that is equal to or lower than several hundred kHz. The power use efficiency thereof is on the order of 60% to 98%. In a case in which electric power is to be supplied over a relatively long range of several meters or more, the electromagnetic wave reception method is employed. The electromagnetic wave reception method allows electric power of several watts or less to be transmitted in a band between medium waves and microwaves. However, the power use efficiency thereof is small. The electric field/magnetic field resonance method has been receiving attention as a method for supplying electric power with relatively high efficiency at a middle range on the order of several meters. Rerated techniques have been disclosed in “A. Karalis, J. D. Joannopoulos, M. Soljacic, “Efficient wireless non-radiative mid-range energy transfer” ANNALS of PHYSICS Vol. 323, January 2008, pp. 34-48”, for example.
FIG. 1 is a diagram showing a wireless power supply system according to a comparison technique. The wireless power supply system 1r includes a wireless power transmitting apparatus 2r and a wireless power receiving apparatus 4r. The wireless power transmitting apparatus 2r includes a transmission coil LTX, a resonance capacitor CTX, and an AC power supply 10r. The wireless power receiving apparatus 4r includes a reception coil LRX, a resonance capacitor CRX, and a load 70.
In such a wireless power supply system 1r, in order to efficiently supply power, there is a need to satisfy the condition for resonance in the wireless power transmitting apparatus 2r and the wireless power receiving apparatus 4r. 
If a foreign substance such as a piece of metal is interposed between the transmission coil LTX and the reception coil LRX when the wireless power supply system 1 is in the resonant state, this leads to a change in the condition for resonance, and the resonant state is disturbed. Also, if the degree of coupling between the transmission coil LTX and the reception coil LRX changes due to movement of the wireless power receiving apparatus 4r, this leads to a change in the condition for resonance, and the resonant state is disturbed. Alternatively, if there is power theft by a malicious user, the resonant state is also disturbed.
The wireless power supply system 1r may preferably be provided with a function of detecting a disturbance in the resonant state. Such an arrangement allows a tuning operation to be controlled according to the detection result, or allows the system to be protected using the detection result, which is convenient.